Treasures as seen under proton irradiation

Silicate materials such as glass, porcelain and ceramics, but also geological findings are characterized by their chemical composition. Corresponding fingerprints of unique pieces - e.g. precious objects of art - have to be studied stringently in a non-destructive manner. Elements along the whole periodic system are of analytical interest. These requirements are met by ion beam analysis (IBA) using protons of 4 MeV energy extracted from vacuum into atmospheric pressure. Atoms of the object of investigation, struck by protons, emit characteristic γ-radiation due to nuclear reactions as well as X-rays via proton-electron interactions. Simultaneous detection of PIGE (Particle Induced Gamma-ray Emission) and PIXE (Particle Induced X-ray Emission) signals allows determination of all chemical constituents. PIGE measures the concentration of the main element Si, consequently the SiO2 content. Moreover, PIGE gives the concentrations of accompanying light elements like Li, B, Na, Mg and Al and their natural oxides; also F can be examined.
Maximum information depths in glass are in the order of 40 µm. Hence, the PIGE results represent the silicate material bulk. PIXE provides concentrations of the elements heavier than Al (Z > 13). Maximum PIXE information depths in glass are only 10 µm, which is due to much higher attenuation of X-rays (E~keV) compared to that of γ-rays (E~MeV). Therefore, the PIXE results stand for the bulk material only if the object surface was not attacked by strong deterioration. PIGE-PIXE analysis of silicate materials must be executed in an interactive manner in order to account for radiation absorption effects. Protons reflected from atoms situated in depth regions near the material surface, i.e. Rutherford Backscattering Spectrometry (RBS), inform of environmental degradation. Thus, IBA is best-suited to calculate both the state of preservation as well as the possible danger of progressive deterioration regarding cultural heritage, especially which made from glass [1].
As is known, K-Ca-silica glass gets decomposed by leaching when being exposed to humid surroundings, even under air conditioning in museum collections. Both, protective storage and preventive conservation have become one of the primary tasks in museum science. The performance of IBA is figuring out glass objects of endangered composition as long as indications of alteration are still not visible. Exhibits sui generis dated from baroque era and treasured inside the museum of applied arts in Dresden were already exposed to the proton beam in air. Several exhibits show visually opacity, some of them a network of fine cracks. The final evaluation of measured data is in the offing. Composition analysis of single obsidian pieces has been a topic of international interest. For their characterization questions have concentrated on fingerprints regarding elements heavier than Fe, being enclosed with low concentrations.
For the bulk materials of interest IBA detection limits are in the order of 5 - 300 µg/g, thus falling for light elements Z < 22 (Ti) below that of X-ray fluorescence spectrometry.

Reference: [1] M. Mäder, C. Neelmeijer, Nucl. Instr. and Meth. B 226 (2004) 110 - 118